1. Field of the Invention
The invention described herein relates to sealing assemblies for effecting a fluid seal between relatively movable parts both in the static and dynamic mode and is particularly directed to sealing a reciprocating piston rod in both high and low pressure conditions.
2. Description of the Prior Art
Ideally, a seal should provide an effective sealing capability under static and dynamic conditions, at both very low and high pressure, with a minimum of breakway friction and running friction and with a long maintenance free service life. Seal assemblies of the type designed as an attempt to meet this ideal, may be typified by that shown in the U.S. Pat. No. 3,848,880 to Tanner issued Nov. 19, 1974 and U.S. Pat. No. 4,053,166 to Domkowski issued Oct. 11, 1977. The devices disclosed therein are believed to be the closest prior art to the applicant's invention. Such seal assemblies are designed to incorporate materials which have good high pressure sealing, friction, wear and extrusion resistance characteristics but poor low pressure sealing ability, with materials which exhibit good low pressure sealing ability but poor friction, wear and extrusion characteristics. Typically, such combination sealing devices will overcome some of the limitations of each seal material but not to a degree that would characterize these combinations as a significant improvement over so called conventional sealing devices. The typical material used to effect the good high pressure sealing characteristics is Teflon, and for low pressure sealing an elastomeric material such as rubber (synthetic or natural) is generally used.
Generally speaking such seals exhibit a number of undesirable qualities because their design relies predominantly on combining the properties of the seal materials and less on seal design. Under static conditions (no movement of the reciprocating element) leakage is a function of the pressure drop across the seal and the dimensions of the spaces between the seal and the surface to be sealed. These spaces are a function of the unit loading and the area of contact. For obvious reasons elastomeric materials require a lower contact force than less resilient materials for the same sealing effect. The same is generally true under dynamic conditions (movement of the reciprocating element such as a rod). However, the spaces between the seal and the surface to be sealed tends to be greater in proportion to the thickness of the fluid film moving under the seal on the surface of the rod. The film thickness is not only a function of the unit loading and contact area but also the rate of pressure rise of the unit loading at the point of initial seal contact with the reciprocating surface. The foregoing is general knowledge.
When designing a rod seal for a typical hydraulic actuator, the seal must be effective at low pressure in the static mode as the rod extends, and at high pressure as the rod retracts. The difficulty lies in providing for enough contact area (footprint) and contact pressure (unit loading) of the elastomeric element of the seal assembly to prevent low pressure static leakage while also ensuring a sharp sealing edge. The sharp sealing edge produces a high rate of pressure rise at the point of initial elastomeric seal contact to minimize fluid film thickness between the seal and rod as the rod extends. Too high a unit loading will blunt the sharp edged seal contact line and cause a low rate of pressure rise and thus less effective leakage control.
Typically, as the pressure on the rod seal rises during rod retraction, the unit loading and area of contact of the elastomer increases which increases friction and wear and shortens service life.
Examples of this dilemma are evident from an examination of Tanner and Domkowski. Tanner uses both diametral interference and high squeeze (unit loading) and a broad contact area (footprint) to ensure low pressure sealing. No sharp line of contact is provided to reduce film thickness as the rod extends. The rubber contributes significantly to the friction of the seal assembly. The inherent large difference between static and dynamic friction levels of the elastomer results in a jerky movemnet of the rod which is unacceptable in close tolerance positional control devices. Service life is materially reduced due to wear during high pressure operation and resultant increased contact area.
Domkowski provides diametral interference of the elastomeric lip element but no radial squeeze at lip contact with the rod in an effort to reduce friction and wear and ensure smooth motion. This design is typical of U-Ring designs with limited low pressure sealing ability. At low pressures, the unit loading and rate of pressure rise at the sealing surface is below the level required for the elastomer to prevent leakage. The movement of a reciprocating element, such as a rod, past the rubber sealing member will therefore drag a thin film of fluid past the rubber seal with the rod. At high pressure, the rubber member is distorted such that even more surface area of the rubber seal is brought into contact with the relatively moving rod. This produces additional and significant friction and contributes prominently to the difference between the static and dynamic friction levels of the seal assembly. The seal is thus less suited for use in close tolerance positional control devices because of the increased jerky operation. Additionally, wear of the elastomer because of high pressure exposure to the moving rod will shorten service life.
It is thus an object of the present invention to provide a seal assembly which forms a superior seal at very low pressures as well as very high pressures and all pressures in between in both the static and dynamic mode.
It is a further object of this invention to provide a low friction, long wearing seal assembly at all presures which results in minimal difference between the static friction and dynamic friction between the seal and moving member.